When did Dems become blue and the GOP red? Jodi Enda notes that before the 2000 election, "there was no uniformity in the maps that television stations, newspapers or magazines used to illustrate presidential elections." Why 2000 changed that:
[T]wo days after voters went to the polls in 2000, both the New York Times and USA Today published their first color-coded, county-by-county maps detailing the showdown between Al Gore and George W. Bush. Both papers used red for the Republican Bush, blue for the Democrat Gore.
"I just decided red begins with ‘r,’ Republican begins with ‘r.’ It was a more natural association," said Archie Tse, senior graphics editor for the Times. "There wasn’t much discussion about it."
Paul Overberg, a database editor who designed the map for USA Today, said he was following a trend: "The reason I did it was because everybody was already doing it that way at that point."
And everybody had to continue doing it for a long time. The 2000 election dragged on until mid-December, until the Supreme Court declared Bush the victor. For weeks, the maps were ubiquitous.